The present invention relates to the field of communications. More particularly, the invention relates to a method for allocating frequency channels between sectors of a cell of a cellular system that minimizes the interference between adjacent sectors.
Wireless telephone networks are widespread and well known in the art. A typical network is a cellular telephone network, that enables a plurality of subscribers to communicate with each other and with subscribers of other telephone networks by communicating with a plurality of base stations, which are actually transceivers (transmitters/receivers), located usually near the center of each cell and transmitting via directional antennas, normally one directional antenna for each sector of the cell. In this network, however, subscriber units normally use omni-directional antennas because of the mobility of each subscriber. In other cases, transceivers of the base stations at each cell may use omni-directional antennas. Co-channel interference in cellular systems that use omni-directional antennas for their base-station transceivers is normally high and is minimized by using only a part of the allocated frequencies in every cell. Co-channel interference in cellular systems that use directional antennas for their base-station transceivers is normally minimized by allocating different frequencies to transceivers that are oriented in the same direction in adjacent cells.
Wireless systems (networks) are often desired in rural areas and developing countries, where the existing capacity of wireline networks is low, and wire installation is of high cost. In addition, it is used anywhere that cabling is not available, or is overly expensive, or where mobility is desired. In these systems, the subscriber units may be mobile, or fixed in location. Since the range of frequency bands that can be used by a wireless system is limited, several methods were developed to enable simultaneous use of channels within the range of frequency bands in the system. Thus, more subscribers can be served using the same range of frequencies. These methods improved the co-channel interference between cells basically by allocating frequency channels according to a plan wherein reuse of frequencies is only allowed in distant sites and/or by obtaining different orientation for positioning the directional antennas of the transceivers at each base station site. Therefore the quality of service of the network subscribers is increased.
In some applications, the transceiver of each base station employs more than one frequency channel to communicate with subscriber units. Directional antennas are used to break up a site, known as a cell, to sectors. This raises a problem of interference between adjacent channels of adjacent sectors. Therefore, basic planning requires that adjacent frequency bands are allocated to non-adjacent sectors.
The flexibility to dynamically move frequency channels from one sector to another is highly desired. For example, if one sector is heavily loaded and another sector is slightly loaded, it is desirable to re-allocate frequency channels, so that frequency band(s) from the slightly loaded sector will be used in the heavily loaded sector, in such a way that a configuration in which adjacent frequency channels appear in adjacent sectors is eliminated.
European Patent Application EP 717577 describes a cellular communication network that employs directional antennas, each of which corresponds to a sector of a cell, and transmits to subscriber units located within the sector. The orientation of antennas, in any adjacent cells, that operate in the same frequency band is different, and therefore the co-channel interference is reduced. However, no flexibility of frequency layout in the cell is discussed, and intrasite interference is not dealt with.
U.S. Pat. No. 5,365,571 describes a cellular radio-telephone system, in which cells are grouped into a plurality of clusters, each of which has an identical number of n adjacent cells. A set of frequencies is assigned to each cell, so that n groups of co-channel cells are formed. Each formed group includes one cell from each cluster, that comprises frequency channels which correspond to the frequency channels for its co-channel cell group. Co-channel sectors in successive co-channel cells are rotated in the clockwise or counter-clockwise direction from a predetermined reference direction, so as to reduce co-channel interference. Again, no flexibility of frequency layout in the cell is discussed, and intrasite interference is not dealt with.
U.S. Pat. No. 5,073,971 describes a cellular radio telephone system with cells that are divided to six sectors, each sector comprises a directional antenna. Frequency channel assignment forms rows corresponding to a two cell reuse pattern according to asymmetrically positioning of the repeating frequency groups, so that one row faces the opposite direction of another row. Therefore, the number of times a frequency can be reused is increased. However, all the above-described patents relate to methods for reducing the co-channel interference resulting from sectors of different cells operating at the same frequency channel. Non of these patents is related to interference between adjacent sectors of the same cell or the transfer of frequency channels from a sector to an adjacent sector.
All the methods described above have not yet provided satisfactory solutions to the problem of minimizing the interference between frequency channels allocated to sector of a cell and frequency channels allocated to other sectors of that cell.
It is an object of the present invention to provide a method for allocating frequency channels to sectors of a cell in a cellular system, which overcomes the drawbacks of prior art.
It is another object of the present invention to provide a method for allocating frequency channels to sectors of a cell in a cellular system, with minimized interference between frequency channels allocated to sector of a cell and frequency channels which are allocated to sectors which are adjacent to this sector.
It is a further object of the present invention to provide a method for allocating frequency channels to sectors of a cell that enable the translation of frequency channels without increasing the interference between adjacent sectors, thereby providing sufficient flexibility in accordance with dynamic system requirements.
Other objects and advantages of the invention will become apparent as the description proceeds.
The present invention is directed to a method for allocating frequency channels between transceivers which transmit to sectors of a cell of a cellular system, with minimal interference between adjacent sectors within the cell. A series of frequency channels, which may be a consecutive series, is allocated to these transceivers, sequentially and alternately between opposing pairs of vertical sectors. Allocation is started from a first pair, and continued, each time, to the next pair which is adjacent to its preceding pair in clockwise or counter-clockwise direction. Preferably, at least one frequency channel from the series of frequency channels is skipped, each time before continuing to allocate frequency channels to the next pair. The order of allocation may also be reversed within sectors forming every other pair. By doing so, identical frequency channels are not allocated twice to the same cell, and adjacent frequency channels are not allocated to the same sector, or to adjacent sectors. Furthermore, adjacent frequency channels do not appear in adjacent sectors even after moving an allocated frequency channel from a sector to each of its adjacent sectors.
Preferably, this allocation method is also suitable for transceivers of cellular systems which employ a frequency hopping technique. The first allocated frequency channel represents the starting point in the hopping cycle for each transceiver. Preferably, sectors of a cell may employ several transceivers, each of which uses a single frequency channel, or a single transceiver that uses several frequency channels for transmission.
The invention is also directed to a cellular system, in which frequency channels are allocated to transceivers which transmit to sectors of a cell, with minimum interference between adjacent sectors within the cell and retain dynamic translations capability of allocated frequency channels from one sector to each of its adjacent sectors.